1. Field of the Invention
The present invention relates to an optical waveguide structure, an optical module and a lens array. Particularly, the present invention relates to an optical waveguide structure used for optical-coupling between an optical element such as a surface-emitting laser, and an optical transmission medium such as an optical fiber. The present invention also pertains to an optical module using the optical waveguide structure, and a lens array having lenses arranged in an array.
2. Description of the Related Art
Surface-emitting lasers are capable of high integration when being arranged in an array and therefore, are excellent in a mounting property on a substrate as compared with edge-emitting lasers. At present, various surface optical elements including surface-emitting lasers are used as key components in optical communication which requires large-capacity transmission or in optical information processing which requires high integration.
For example, in a down-sized optical radio transceiver, surface-emitting lasers are mounted on a built-in substrate in an array. Further, laser light emitted from the surface-emitting lasers is received by optical fibers arranged in an array in parallel with the substrate. In this case, a traveling direction of light within a module must be bent at 90°. For realizing this technique, the following optical system is conventionally proposed. That is, a mirror is obliquely arranged within a propagation path of light to bend the traveling direction of laser light emitted from the surface-emitting lasers so that the laser light can be optically coupled to optical fibers. However, the mirror used for the sake of the object is required to have high flatness or low surface roughness and therefore, manufacture of the mirror is not necessarily easy.
Conventionally, there is also proposed a technique of using an optical waveguide structure in place of the mirror. The structure includes a curved surface of which the cross section is curved in a circular arc shape and on which a propagation path (optical waveguide) of light is formed. In the structure, optical-coupling between surface optical elements and optical fibers, which are arranged almost orthogonally to each other, can be performed (see, e.g., Japanese Unexamined Patent Publication No. 2005-115346). In this proposal, the need for manufacture of mirrors and the need for an optical axis matching of components including the mirrors are eliminated by using the optical waveguide structure. Thus, facility of fabrication of optical modules is attained.
In the manufacture of optical modules, when assembling elements constituting the modules, such as an optical element, an optical fiber, or an optical waveguide structure provided between the optical element and the optical fiber, positioning of the elements becomes very important for obtaining a high optical-coupling efficiency.
With respect to the positioning in assembling the optical modules, for example, the following method is conventionally proposed. That is, when mounting an end-face light receiving-type optical element on a substrate where an optical waveguide is partially formed, an insulating layer is formed between the optical element and the substrate in order to adjust heights of optical axes in an optical waveguide core and an optical element active layer. Thus, a height of the optical element active layer from the substrate is controlled (see, e.g., Japanese Unexamined Patent Publication No. 2001-108871). Further, also the following method is proposed. That is, a tapered optical waveguide formed by using a silicone resin is sandwiched between an end light emitting type optical element and an optical fiber. Further, connection guides are provided on respective components to allow the components to be placed opposite and connected to each other. Thus, a relative displacement of the respective components or shape distortion in terminal areas accompanying a temperature change is suppressed to attain reduction in light loss (see, e.g., U.S. Pat. No. 3,059,171).
In addition to the above-described methods, there is considered a method where emitted light from an optical element or incident light to an optical element is focused using a lens. In this case, an interval between the optical element and the lens must be controlled near a focal length of the lens.
For the method for controlling the interval, for example, there is considered the following method. That is, surface optical elements are arranged on a substrate in an array. Further, an optical waveguide structure having lenses arranged in an array is used. A projection structure is previously formed on this optical waveguide structure. The projection is brought into contact with the substrate on which the surface optical elements are mounted. Thus, the intervals between the surface optical elements and the lenses are controlled to an interval according to a length of the projection. Alternatively, there is also considered the following method. That is, a spacer is provided between the optical waveguide structure and the substrate. Thus, the intervals between the surface optical elements and the lenses are controlled to an interval according to a thickness of the spacer.
However, even when using these methods, the following problem occurs. That is, when warpage occurs in the substrate itself or dispersion occurs in the thickness of a heat sink or adhesive layer which may be provided between the substrate and the optical elements, it is difficult to control the intervals between the optical elements and the lenses with high accuracy.
Further, the same problem may occur also in the case of controlling the intervals between the optical waveguide end faces of the optical waveguide structure and optical elements in the optical module using the optical waveguide structure, for example, in the optical module where light is directly exchanged between the optical waveguide structure and the optical elements without passing through the lenses, in addition to the case of controlling the intervals between the optical elements and the lenses as described above.